The primary aim of this project is to determine if (i) apoptosis of the cycling pool, or (ii) cell cycle arrest is contributing to the failure of CD4 cells to regenerate, and (iii) identify the potential cellular lesions associated therewith. Towards this goal, the investigators will focus on longitudinal samples obtained during the acute phase of anti-viral therapy. They will determine by FACS analysis the presence or intensity of cytokine receptors, adhesion molecules, apoptotic markers, DNA content, and restriction point-related cyclins on Ki67-expressing CD4 cells and their ex vivo cytokine response. By comparing the data obtained at time points which display different kinetics of CD4 lymphocyte regeneration, they hope to better define the conditions which potentiate or inhibit peripheral CD4 expansion.